The present invention relates to oscillators having low phase noise, and particularly to methods and arrangements for providing radio frequency signals having lower phase noise than heretofore available using techniques of the art.
In many radio frequency systems, including phase modulated digital communications systems, it is desired to use signal sources at transmitters and receivers which provide carrier signals with low phase noise. Conventionally phase noise is controlled to a low level by careful design of the primary reference signal source, e.g. a crystal oscillator. Such oscillators usually provide a signal at a reference frequency such as 10 MHz which is then processed in a frequency synthesizer to provide a local oscillator signal at the desired carrier frequency with frequency stability derived from the reference oscillator by techniques well known in the art.
U.S. Pat. No. 4,551,689 to Scala, et al. describes a system wherein phase noise arising in the synthesizing process is improved by providing an arrangement of crystal oscillators. This arrangement can effectively reduce the phase noise of a noisy signal, but can do so only to the level of the noise of the crystal oscillators provided. As mentioned in the Scala, et al. patent, phase noise is generally increased when a signal is multiplied to a higher frequency. The increase in phase noise resulting from multiplication is given in dB. as 20 log K where K is the frequency multiplication ratio. Likewise when the frequency of a carrier signal is reduced by a frequency divider the phase noise is reduced by the same factor.
The present inventor has discovered a method and arrangement which enables an increase in carrier frequency with a phase noise increase of less than 20 log K whereby in subsequent division the phase noise of a signal can be reduced to below its original level.
It is therefore an object of the present invention to provide a method and apparatus for reducing phase noise in a carrier signal to levels below those previously available and to provide apparatus for carrying out that method.